Building a Higgs Factory!

by Nicholas Mee on June 13, 2013

The Large Hadron Collider is currently undergoing a major upgrade that will enable it to reach its full design energy when restarted towards the end of next year. Meanwhile physicists have released plans for the next generation collider that will take over from the LHC in around a decade’s time.

The Japanese proposal for the International Linear Collider.(copyright Shigemi Numazawa)

The Alpha Male of Nuclear Physics

It is just over a century since Ernest Rutherford first used high energy alpha particles emitted by the radioactive element radium to probe the structure of matter. (An alpha particle is identical to the nucleus of a helium atom.) Rutherford began these experiments at McGill University in Montreal and continued them when he became department head at Manchester University. In Manchester, under Rutherford’s guidance, Hans Geiger and Ernest Marsden undertook an experiment in which these bullet particles were fired at wafer-thin gold foil.

Gold Nuggets

From their results Rutherford deduced the existence of a very massive, but tiny nucleus at the heart of the atom. Rutherford continued with his alpha particle bombardment and ten years later in 1919 he revealed the proton, one of the particles from which atomic nuclei are composed. Within a single decade Rutherford had made two of the most important discoveries in the history of science, neither of which would win him a Nobel prize. (Rutherford had already been awarded the Nobel Prize in Chemistry in 1908.)

The Cockcroft-Walton machine is now in the Science Museum in London.

The Cockcroft-Walton Machine

Rutherford realized that there were limits to what could be achieved with his alpha particle probes. To investigate the structure of matter further it would be necessary to devise a means of accelerating particles to much higher energies before smashing them into the material under investigation.

This was first achieved in 1932 by John Cockcroft and Ernest Walton, two physicists in Rutherford’s department at Cambridge University. The Cockcroft-Walton machine is a stunning piece of equipment. It looks like a device from a 1930s science fiction comic. The machine could accelerate protons to an energy that was just sufficient to enter an atomic nucleus and trigger various nuclear reactions, such as the disintegration of a lithium nucleus into two alpha particles. Cockcroft and Walton were awarded the Nobel Prize in Physics in 1951 for the first artificial transmutation of the elements.

The Large Hadron Collider

Particle accelerators have come a long way in the 80 years since the Cockcroft-Walton machine. The energy of a particle is usually quoted in electron Volts. One electron Volt is the energy that an electron (or any other charged particle) gains when travelling around a one Volt electrical circuit. The Cockcroft-Walton machine could accelerate protons to 500,000 electron Volts. By comparison, the Large Hadron Collider has been accelerating protons to an energy of 4 TeV before colliding them head-on. TeV stands for Tera electron Volts, where Tera means trillion. So the protons in each beam of the LHC have 8 million times the energy of Cockcroft and Walton’s protons. When the LHC restarts next year this energy will almost double to 7 TeV per beam and set another new world record.

What Comes Next?

The big discovery of the LHC is, of course, the Higgs boson. But finding evidence for the Higgs in the detectors of the LHC is a bit like looking for a tiny needle in a gargantuan haystack. The LHC collides beams of protons. Long after Rutherford’s time physicists discovered that protons are not simple particles. They are composed of smaller particles known as quarks and gluons. This means that when two extremely high energy protons collide the resulting debris is very messy.

Computer reconstruction of the particle debris following a collision event in the CMS detector at the LHC.(copyright CERN, Geneva)

A Higgs Factory

Physicists have been investigating the potential for a new type of machine that would generate very clean collisions and produce Higgs bosons in large quantities. The aim would be to create a ‘Higgs Factory’. The machine would collide electrons with their antiparticles which are known as positrons. Electrons and positrons have no subcomponents, so this is like firing magic bullets rather than a spray of shotgun pellets. It would also be possible to tune the energy of the beams to the mass of the Higgs boson and thereby produce it in abundance.

Generating large quantities of Higgs bosons will be vital in determining its properties. Physicists need to know how the Higgs decays to see whether it really is responsible for giving electrons and quarks all their mass and to see whether it satisfies all the other predictions of the standard model. If there are any surprises then they will be great clues for finding new theories that go beyond the standard model and enter the realm of uncharted new physics.

The International Linear Collider

On 12 June a five-volume report was published containing a blue-print for this new machine, which is known as the International Linear Collider (ILC). It has been designed to complement and advance the work of the LHC. The idea is to build two linear accelerators and align them head to head. One will accelerate a beam of electrons, the other will accelerate a beam of positrons. The two beams will be fired directly at each other and will collide at the centre of the machine.

The design is for a 31 kilometre machine that will collide bunches of electrons and positrons 7,000 times a second. Although thinner than a human hair, each bunch will contain around 20 billion electrons or positrons that have been accelerated to an energy of 250 GeV (billion electron Volts). The report concludes that all the necessary technology is currently available. All that is required now is the political will to proceed, and a decision to start construction may not be too far off as there are encouraging signs that Japan will bid to host the machine.

Incidentally, should you think that the development of these machines is rather frivolous, it is worth noting that the vast majority of the world’s particle accelerators are now found in hospitals and are routinely used for radiation therapy to treat cancer. Only the most powerful accelerators are involved in particle physics research.

Compact linear accelerator designed for the treatment of cancer in hospital.

The article is very lucid and educative even to professionals. I enjoyed reading this article. This article has given me a very clear point of view of the state of art of finding new particle. I was still wondering what is after confirming the existence of Higgs boson. What may be the consequences?